The objective of the thesis consisted in development and verification of the methods for determination of earthquake source mechanisms and stress field from earthquake focal mechanisms in the Transcarpathian region of Ukraine.
Determination of present-day stress state of earth’s crust based on data on earthquake mechanisms is considered important and actual problem, remaining in the focus of scientific interest of many researchers for a long time now.
At the current stage of seismological investigations, the solution to the problem of determination and interpretation of earthquake source parameters demands introduction of novel approaches, based on theoretical as well as experimental analysis of data. This especially relates to the region of Ukrainian Transcarpathians, with its relatively low level of local seismic activity and small number of seismic stations.
Basic data on the earthquakes (time, coordinates, depth) were acquired in the thesis from the seismological bulletin of Ukraine and reports provided by Department of Seismicity of Carpathian Region, Subbotin Institute of Ge-ophysics, National Academy of Sciences of Ukraine. Two methods were em-ployed for determination of focal mechanisms. In the most part, the mecha-nisms were determined by graphic method from polarities of first motions of P-waves at the stations, a method although considered as traditional but still as the most reliable one. When data on polarities of first motion proved insufficient, it was proposed to employ the inversion of waveforms. In total, focal mechanisms of 50 earthquakes were determined which occurred in the region during a period from 2012 to 2015. Mechanism parameters of the 23.10.2006 MS3.7 Berehove earthquake were taken from literature data.
Analysis of waveforms had shown that 30 out of 50 earthquakes belonged to a category of the so called recurrent (or similar) earthquakes and a conclusion was drawn out that since they comprise such a significant fraction of all earthquakes recorded in the region they may be considered as a significant feature of local seismicity.
Comparative analysis of source mechanisms determined by graphic method and by inversion of waveforms was performed. Subsequently, the data on mechanisms were used in determination of stress field.
Further in the thesis, analysis of present methods for determination of stress field from focal mechanisms of earthquakes was done. The conclusion was drawn out that there was much in common between them. In particular, these methods usually assume that (1) tectonic stress is uniform (homogene-ous) in the study area, (2) earthquakes occur on pre-existing faults with vary-ing orientations and (3) the slip vector points in the direction of shear stress on the fault (the so called Wallace-Bott hypothesis).
In the thesis, Michael’s method was employed for determination of stress tensor. To resolve the ambiguity of which of the nodal planes corresponds to real rupture it was proposed to apply a so called fault instability constraint. Stresses were no longer calculated in one step as in Michael’s original method but in iterations. The iterations were needed to identify the fault planes and thus to determine a more accurate stress field. The susceptibility of a fault plane to failure was estimated using a Mohr-Coulomb failure criterion, describing a relation between shear and normal stresses that will cause a fracture. According to this criterion, shear traction τ on an activated fault must exceed a critical value τc. At the same time, it also was proposed to use the fault instability coefficient I (the fault plane instability criterion) for identification of the fault plane.
In the thesis, for the first time a study was conducted in which the stress filed in the seismically active region of Transcarpathians was determined from local earthquake source mechanisms. Principal stresses 321,,σσσ were determined using more than 50 earthquakes with magnitudes between 1 and 3.7 which had occurred in the Transcarpathians during a period between 1.06.2012 and 06.08.2015. Corre-lation of mechanisms was conducted with active faults and stress fields were determined by two methods – method of P-T axes and Vavrycuk’s modification of Michael’s method – which has enabled to validate their present day kinematics. General statistics and typification has been performed enabling to identify the character-istic features of stress-strained state of earth’s crust across the whole region. Depended on orientation of kinematics axes the focal mechanisms have been attributed to three groups, corresponding to thrust, and strike and fault types. Mechanisms of strike and thrust type have appeared as the most common, although thrust compo-nent of slip was present in almost all strike mechanisms.
Key words: earthquake source, focal mechanism, nodal plane, stress field, principal stress, Mohr-Coulomb criterion, method of P-T axes, Michael’s method, typification of mechanisms.
